Containment of contaminants is a recognized need, and the use of vertical barrier walls of low permeability has been proposed for that purpose. See, for example, EPA HANDBOOK: Remedial Action at Waste Disposal Sites (Revised), EPA/625/6-85/006 (PB87-201034), Chapter 5. Also, Mclaughlin et al., in U.S. Pat. No. 3,375,872, disclose the use of injection and extraction wells to inject low pH (&lt;1.5) silicic acid solutions into oil field zones of high permeability to water ("thief" zones), so as to plug those zones and thereby inhibit interference with successful secondary recovery water flood injection procedures. Mclaughlin et al. reported that controlling the acidity of the silicic acid solution provided control over the setting or gelling time so as to provide adequate time for the grout solution to be introduced into the thief zones. In another example, Bennett et al., in U.S. Pat. No. 4,732,213, disclose a process for plugging permeable zones in subterranean formations or subterranean leaks which comprises injecting an aqueous solution of a nonaggregated colloidal silica sol, and causing the solution to gel in the subterranean formation.
However, known containment techniques do not create any underlying barrier to provide downward control, or what might be termed a "floor". The need for such an confining layer or impermeable floor sealed to the perimeter wall becomes obvious when various situations are considered. For example, when the water table within a substantially impermeable vertical perimeter is above that of the surroundings, hydrostatic pressure will drive contaminated water from within the contaminated zone below the perimeter wall in the form of a plume free to move horizontally or down into other groundwater strata or aquifers. A second situation exists when a zone is contaminated by a water-immiscible liquid which is denser that water (such contaminants are frequently referred to as Dense Non-Aqueous Phase Liquids or DNAPLs). Chlorinated hydrocarbons, such as tetrachloroethylene, are examples of DNAPLs. Such dense contaminants may flow downwards under gravity to contaminate a lower aquifer even if there is no flow of groundwater downward from the contaminated zone to the lower aquifer. The site topography can present yet another limitation to the use of vertical containment barriers. A vertical slurry wall can be keyed into an underlying geological layer. But in order for the underlying geological layer to function as a confining layer (floor), it must be: of sufficiently low permeability adequately to retard downward migration of the contaminant; of sufficient thickness to permit formation of an adequate key (typically 2-3 feet); at a depth of less than about 70 feet if costs are not to increase to an unacceptable level; and continuous beneath the contaminated zone.
Contaminated sites where there is no naturally occurring underlying layer which serves as a barrier to downward migration of contaminants meeting all of the foregoing requirements are common. By way of example, three common naturally-occurring situations in which the lower confining layer is inadequate are situations in which the contaminated site is underlain by: sand or other strata of high permeability; a discontinuous impermeable layer, as would exist with a discontinuous layer of clay (often termed clay lenses due to the characteristic vertical section of the clay deposits); or by a layer of fractured rock wherein the fractures provide channels through what otherwise might be a satisfactorily impermeable confining layer.
Man-made underlying barriers are in use in sites containing contaminants; e.g. that is normally so in a properly constructed hazardous landfill, where a floor of heavy duty plastic sheeting over a constructed, compacted, and impervious layer of clay may be laid down during construction. However, such a floor can not be built beneath pre-existing contaminated zones without first removing the contaminants, a formidable undertaking involving extensive, expensive, and possibly hazardous excavation. It is thus a technique to be avoided, if at all possible.